1. Field of the Invention
The present invention relates to subterranean well cementing operations, and more particularly, to viscosifying and fluid loss controlling additives for use in well cements subjected to temperatures up to 500xc2x0 F., well cement compositions containing the additives and methods of using the compositions.
2. Description of the Prior Art
Hydraulic cement compositions are commonly utilized in subterranean well completion and remedial operations. For example, hydraulic cement compositions are used in primary cementing operations whereby strings of pipe such as casings and liners are cemented in well bores. In performing primary cementing, a hydraulic cement composition is pumped into the annular space between the walls of a well bore and the exterior surfaces of a pipe string disposed therein. The cement composition is permitted to set in the annular space thereby forming an annular sheath of hardened substantially impermeable cement therein which supports and positions the pipe string in the well bore and bonds the exterior surfaces of the pipe string to the walls of the well bore. Hydraulic cement compositions are also utilized in remedial cementing operations such as plugging highly permeable zones or fractures in well bores, plugging cracks or holes in pipe strings and the like.
Cement composition viscosifying additives are often used in well cement compositions to prevent settling of solids in the cement compositions after they are placed in a subterranean zone to be cemented. While such cement composition viscosifying additives have been utilized successfully at subterranean temperatures up to about 350xc2x0 F., at higher temperatures the heretofore utilized viscosifying additives have not been able to prevent thermal thinning which results in the settling of solids in cement compositions. The settling of the solids in a cement composition results in defective cementing and failure of the set cement to provide zonal isolation.
Fluid loss control agents are also used in well cement compositions to reduce fluid loss from the cement compositions to permeable formations or zones into or through which the cement compositions are pumped. In primary cementing, the loss of fluid, i.e., water, to permeable subterranean formations or zones can result in premature gelation of the cement composition whereby bridging of the annular space between the permeable formation or zone and the pipe string being cemented therein prevents the cement composition from being placed over the entire length of the annulus.
Thus, there are needs for an improved viscosifying additive for well cements which can be utilized in cements subjected to temperatures up to 500xc2x0 F. and which also provides fluid loss control to the cements at such temperatures, improved well cement compositions containing the additive and methods of using the cement compositions.
The present invention provides an improved viscosifying additive for use in well cement compositions subjected to temperatures up to 500xc2x0 F. which also provides fluid loss control to the cement compositions, well cement compositions including the additive and methods of using the cement compositions which meet the needs described above and overcome the deficiencies of the prior art.
The viscosifying and fluid loss controlling additive of this invention for use in well cements at temperatures up to 500xc2x0 F. is basically comprised of a mixture of a polymer and a homopolymer. The polymer is comprised of at least one monomer which is calcium tolerant, anionic and disperses basic cement slurries, at least one monomer which hydrolyzes in basic cement slurries to generate anionic carboxylate groups that bind with calcium and viscosify the slurries and at least one monomer which generates non-ionic pendant groups on the polymer upon hydrolyzing in basic cement slurries to prevent polymer precipitation, and a homopolymer of a monomer which hydrolyzes in basic cement slurries to generate anionic carboxylate groups that bind with calcium, viscosify the slurries and prevent settling in the slurries. The polymer preferably has a molecular weight in the range of from about 300,000 to about 1.5 million and the homopolymer preferably has a molecular weight in the range of from about 900,000 to about 1.5 million. The polymer is present in the mixture in an amount in the range of from about 50% to about 95% by weight of the mixture and the homopolymer is present therein in an amount in the range of from about 5% to about 50% by weight of the mixture.
The improved well cement compositions of this invention are basically comprised of a hydraulic cement, water present in an amount sufficient to form a slurry and a viscosifying and fluid loss controlling additive comprised of a mixture of a polymer and a homopolymer as described above.
The viscosifying and fluid loss controlling additive can be added in particulate solid form directly to the hydraulic cement or mix water utilized or it can be combined with water whereby a storable aqueous solution is formed which can be conveniently combined with the cement composition mix water.
The methods of this invention for cementing a zone in a subterranean formation penetrated by a well bore are basically comprised of the steps of preparing a cement composition of this invention including a viscosifying and fluid loss controlling additive comprised of a mixture of a polymer and a homopolymer as described above, placing the cement composition in the zone to be cemented and allowing the cement composition to set into an impermeable solid mass therein.
It is, therefore, a general object of the present invention to provide an improved high temperature viscosifying and fluid loss controlling additive for well cements, well cement compositions and methods of using the cement compositions.
Other and further objects, features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of preferred embodiments which follows.